(1) Field of the Invention
The invention relates to a process for the synthesis of zeolites with an aluminosilicate skeleton belonging to the faujasite structural group. It further relates to the products obtained and to their application in adsorption and catalysis.
(2) Background Art
Zeolites are crystalline tectosilicates. The structures consist of assemblies of TO.sub.4 tetrahedra forming a three-dimensional skeleton by sharing oxygen atoms. In zeolites of the aluminosilicate type, which are the most common ones, T denotes tetravalent silicon and trivalent aluminium. The abovementioned three-dimensional skeleton exhibits cavities and channels which have molecular dimensions and accommodate cations compensating the charge deficiency linked with the presence of trivalent aluminium in TO.sub.4 tetrahedra, the said cations being generally exchangeable.
As a general rule, the composition of zeolites may be denoted by the empirical formula (M.sub.2/n O.Y.sub.2 O.sub.3.xZO.sub.2) in the dehydrated and calcined state. In this formula Z and Y denote the tetravalent and trivalent elements of the TO.sub.4 tetrahedra respectively, M denotes an electropositive element of valency n, such as an alkali metal or alkaline earth metal, and constitutes the compensating cation, and x is a number which can vary from 2 to theoretically infinity, in which case the zeolite is a silica.
Each type of zeolite has a distinct microporous structure. The variation in the dimensions and shapes of the micropores from one type to another results in changes in the adsorbent properties. Only the molecules which have certain dimensions and shapes are capable of entering the pores of a particular zeolite. Because of these remarkable characteristics, zeolites are very particularly suitable for the purification or separation of gaseous or liquid mixtures, such as, for example, the separation of hydrocarbons by selective adsorption.
The chemical composition, including in particular the nature of the elements present in the TO.sub.4 tetrahedra and the nature of the exchangeable compensating cations, is also an important factor involved in the selectivity of the adsorption, and above all in the catalytic properties of these products. They are employed as catalysts or catalyst supports in the cracking, reforming and modification of hydrocarbons, and in the conversion of many molecules.
Many zeolites exist in nature; these are aluminosilicates whose availabilities and properties do not always correspond to the requirements of industrial applications. Consequently, the search for products which have new properties has led to the synthesis of a large variety of zeolites, among which there may be mentioned zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244) and zeolite Y (U.S. Pat. No. 3,130,007).
Zeolites of the faujasite structural group are characterised by a three-dimensional skeleton structure which can be described by means of the assembly of modules called cube-octahedra. Each of these modules consists of 24 tetrahedra containing the elements Si and Al in our case and bridged by oxygen according to the principle described above. In the cube-octahedron, the tetrahedra are linked so as to form eight rings containing six tetrahedra and six rings containing four tetrahedra.
Each cube-octahedron is joined, with tetrahedral coordination, via four rings containing six tetrahedra, to four neighbouring cube-octahedra.
To show the relationships which unite the various members of the structural group it is convenient to consider the structural planes in which the cube-octahedra are arranged at the vertices of a plane network of hexagons. Each cube-octahedron is thus connected to three neighbours in the structural plane.
The fourth connecting direction is directed alternately on each side of the structural plane and enables the cube-octahedra to be connected between neighbouring and parallel structural planes.
All the solids belonging to the faujasite structural group have interconnected channels approximately 0.8 nm in diameter. Thus, faujasite is a zeolite with an aluminosilicate skeleton whose structure corresponds to the stacking of three distinct structural planes ABC corresponding to a structure of cubic symmetry.
Compounds of the same structure as faujasite can be obtained by synthesis from an aluminosilicate gel.
The general process of synthesis of zeolites with an aluminosilicate skeleton belonging to the faujasite structural group consists of a hydrothermal crystallisation of sodium aluminosilicate gels of particular compositions and containing a structuring agent consisting of a metal cation.
More precisely, a process of this kind consists in producing first of all a reaction mixture which has a pH higher than 10 and contains water, a source of tetravalent silicon, a source of trivalent aluminium, a source of hydroxide ions in the form of a strong base, a source of M.sup.n+ metal cations, n being the valency of M, so as to obtain an aluminosilicate gel which has the desired composition to permit its crystallization into a compound of the faujasite structural group, and in then maintaining the gel obtained directly or after prior maturing, at a temperature not exceeding 150.degree. C. and at a pressure which is at least equal to the autogenous pressure of the mixture consisting of the said gel for a sufficient period to effect the crystallisation of this gel.
As indicated earlier, a process of this kind does not make it possible to synthesise zeolites with an aluminosilicate skeleton having the faujasite structure of cubic symmetry and an Si/Al ratio higher than 3.
It has now been found that certain organic molecules belonging to the class of polyalkylene oxides have the property of directing the crystallization of aluminosilicate gels towards zeolites of the faujasite structural group, which are characterised by Si/Al ratios which may be higher than 3. These molecules introduce a pronounced stabilising effect which makes it possible to decrease the concentration of the hydroxide ions in the synthesis medium, which results in obtaining a higher Si/Al ratio and a substantial improvement in the yield.